Endocrine System & Hormone Vocabulary

Overview of Hormones & the Endocrine System

  • Stereotype-buster: “hormonal” ≠ only sex hormones; >50 distinct chemical messengers circulate from birth to death.
  • Hormones = blood-borne, long-acting signals that complement the nervous system’s fast electrical messages.
    • Nervous system: action potentials along neurons → millisecond onset, millisecond offset.
    • Endocrine system: hormones through bloodstream → seconds-to-hours onset, minutes-to-days duration.
  • Prime mission: maintain overall homeostasis (temperature, blood chemistry, stress handling, growth, reproduction, immunity, sleep, metabolism, etc.).
  • Glands are small, scattered “rogues” (brain → throat → abdomen → gonads). Any structure that secretes a hormone qualifies as a gland.
  • Two functional “bosses” in the body:
    • Nervous system (electro-chemical wiring)
    • Endocrine system (chemical broadcasting)
  • Many hormones form cascades: Hormone 1 → triggers Hormone 2 → triggers Hormone 3 … (“chemical relay race”).

Hormone Chemistry & Solubility

  • Two major chemical families:
    • Amino-acid based (peptides, proteins, amines) → usually water-soluble.
    • Steroid / lipid-derived (e.g. cholesterol-based) → lipid-soluble.
  • Solubility dictates receptor location:
    • Water-soluble → cannot cross lipid membrane → bind extracellular receptors.
    • Lipid-soluble → diffuse through membrane → bind intracellular / nuclear receptors.
  • Binding alters target-cell activity (↑ or ↓ gene transcription, enzyme rates, membrane transport, etc.).

Major Glands, Locations & Signature Hormones

  • Hypothalamus (diencephalon floor)
    • “Neuro-endocrine boss”; monitors blood, issues releasing/inhibiting factors (TRH, CRH, GnRH, GHRH, etc.).
  • Pituitary (sits in sphenoid’s sella turcica, attached by infundibulum)
    • Anterior (adenohypophysis – glandular):
      • TSH, ACTH, FSH, LH, GH, PRL, MSH.
    • Posterior (neurohypophysis – neural): stores/secretes ADH & OT.
  • Pineal (epithalamus, posterior to thalamus)
    • \text{Melatonin} → circadian rhythm / sleep–wake.
  • Thyroid (anterior neck, over trachea; butterfly-shaped)
    • Follicular cells → T3 (triiodothyronine) & T4 (thyroxine): set metabolic rate.
    • Parafollicular (C) cells → \text{Calcitonin}: ↓ blood Ca^{2+}.
  • Parathyroids (4 pea-sized glands on thyroid’s posterior)
    • PTH: ↑ blood Ca^{2+} by bone resorption, kidney reabsorption, vit D activation.
  • Thymus (superior mediastinum; large in infants, atrophies post-puberty)
    • \text{Thymosin} & \text{Thymopoietin}: mature T-lymphocytes.
  • Adrenal glands (on kidney poles)
    • Capsule → Cortex → Medulla.
    • Cortex zones (superficial → deep):
      • \text{Zona Glomerulosa} → mineralocorticoids (e.g., \text{Aldosterone}) – Na^+/K^+ & pH balance.
      • \text{Zona Fasciculata} → glucocorticoids (e.g., \text{Cortisol}) – stress, glucose, inflammation.
      • \text{Zona Reticularis} → gonadocorticoids (androgens/estrogens).
    • Medulla (neuro-endocrine) → \text{Epinephrine} & \text{Norepinephrine} (fight-or-flight catecholamines).
  • Pancreas (retro-peritoneal; dual function)
    • Endocrine – Islets of Langerhans:
      • \alpha-cells → \text{Glucagon}: ↑ blood glucose.
      • \beta-cells → \text{Insulin}: ↓ blood glucose.
    • Exocrine – acinar cells → digestive enzymes & bicarbonate.
  • Gonads
    • Ovaries → \text{Estrogen} & \text{Progesterone}: secondary female traits, uterine cycle, bone health.
    • Testes (interstitial/Leydig cells) → \text{Testosterone}: male traits, spermatogenesis, muscle/bone growth.

Classic Regulatory Examples & Cascades

Blood-Glucose Control (Pie Example)

  • Eat strawberry-rhubarb pie à la mode → blood glucose spikes.
    • Pancreatic \beta-cells release \text{Insulin} → glucose stored as glycogen/fat → \downarrow blood glucose.
  • Skip food for hours → blood glucose falls.
    • \alpha-cells release \text{Glucagon} → glycogen breakdown & hepatic gluconeogenesis → \uparrow blood glucose.
  • Dysregulation → diabetes mellitus (hypo-insulin) or insulinoma (hyper-insulin).

Hypothalamic–Pituitary–Adrenal (HPA) Axis – Stress

  1. Stress signal (e.g., waking in burning house). Hypothalamus secretes CRH.
  2. Anterior pituitary releases ACTH.
  3. Adrenal cortex secretes cortisol + mineralocorticoids →
    • ↑ BP, ↑ blood glucose, ↓ immunity, ↓ reproduction.
  4. Negative feedback: rising cortisol → inhibits CRH & ACTH release.
  • Endocrine limb is slower to rise & fall than sympathetic nerve firing → lingering “post-stress” feeling.

Hypothalamic–Pituitary–Thyroid (HPT) Axis – Metabolic Thermostat

  • Cold blood reaches hypothalamus → TRH → anterior pituitary TSH → thyroid releases T3/T4.
  • T3/T4 enter nuclei of nearly every cell → ↑ glucose catabolism → ↑ ATP → heat (calorigenic effect).
  • Rising T3/T4 feedback inhibits TRH/TSH once warmth restored.

Key Disorders & Clinical Correlates

  • Hyper- / Hypo-secretion extremes underpin most endocrine diseases.
  • Graves’ Disease (Marie’s case study)
    • Autoantibodies mimic TSH → thyroid over-produces T3/T4 (hyperthyroidism).
    • Symptoms: weight loss, heat intolerance, anxiety, insomnia, palpitations, exophthalmos (bulging eyes—orbital inflammation).
    • Treatment: anti-thyroid drugs, radioactive iodine, surgery.
  • Diabetes Mellitus
    • Type 1: autoimmune destruction of \beta-cells → no insulin.
    • Type 2: insulin resistance → compensatory hyperinsulinemia → eventual \beta failure.
  • Hyperparathyroidism: excess PTH → bone demineralization, kidney stones (author’s own elevated PTH example → potential low Ca²⁺ intake).

Anatomical Landmarks & Histology Snapshots

  • Sella turcica (“Turkish saddle”): sphenoid depression housing pituitary.
  • Infundibulum: stalk linking hypothalamus & pituitary.
  • Thyroid follicle: colloid-filled sphere lined by simple cuboidal follicular cells; parafollicular C-cells in between.
  • Adrenal cortex layered mnemonic: “Salt → Sugar → Sex” (Glomerulosa, Fasciculata, Reticularis).
  • Pancreatic islet resembles pale-staining “mini-island” amid darker acini.

Nervous–Endocrine Interplay & Integration

  • Hypothalamus = anatomical & functional bridge (neurons + hormone secretion).
  • Sympathetic nervous system premotor signals can drive adrenal medulla secretion.
  • Many target organs express both neurotransmitter & hormone receptors (redundancy & fine-tuning).

Real-World, Ethical & Practical Notes

  • OTC melatonin exploited as sleep aid; chronic misuse may distort circadian cues.
  • Performance-enhancing misuse of anabolic steroids (synthetic androgens) disrupts natural gonadocorticoid axis.
  • Public stereotype reduction: mood swings, acne, pregnancy tears ≠ “just hormones” — endocrine network is multidimensional & omnipresent.
  • Maintaining endocrine health: balanced nutrition (iodine, Ca²⁺, vit D), stress management, regular screening (thyroid panel, glucose, bone density).

Learning Aids Referenced in Lecture

  • Guided worksheets (“practice” points not graded but highly recommended).
  • Flash-card sets embedded in module for quick recall of gland → hormone → function.
  • “Operation” drag-and-drop game: label glands/hormones, reinforce anatomy.
  • Mnemonics: “FLAT PG” (FSH, LH, ACTH, TSH, Prolactin, GH), “Go Find Rex” for adrenal cortex layers.

Numerical / Symbolic Quick Sheet (LaTeX Notation)

  • T_4 = Thyroxine (4 iodine atoms)
  • T_3 = Tri-iodothyronine (3 iodine atoms)
  • Ca^{2+} homeostasis: Calcitonin (↓) vs $PTH (↑)
  • Blood glucose feedback:
    • Eat → \beta-cell: \uparrow Insulin → \downarrow [Glucose]
    • Fast → \alpha-cell: \uparrow Glucagon → \uparrow [Glucose]
  • Adrenal cortex priority mnemonic: \text{G} > \text{F} > \text{R} (external → internal)

Bottom-Line Takeaways

  • “We are hormonal all the time.” Every cell and every second involves endocrine influence.
  • Cascades & negative feedback form self-correcting loops essential for life.
  • Disruption—whether autoimmune, tumor, nutritional, or iatrogenic—destabilizes homeostasis and underlies a wide spectrum of disease.
  • Respect the hormones: they quietly keep you “not dead.”